Chalcogenide electrochemical cell

ABSTRACT

A battery is provided in which the anode contains an alkali metal in a high state of thermodynamic activity; the cathode comprises a partially alkali metal-intercalated chalcogenide of the formula A y  MZ x  wherein A is an alkali metal more electropositive and larger than the anode alkali metal, M is a transition metal of Group IV or V, x is a numerical value of from about 1.8 to about 2.1, y is a numerical value of from about 0.01 to about 1 and Z is sulfur, selenium or tellurium; and the electrolyte comprises ions of the anode metal in a medium which is compatible with the anode and cathode allowing transport of the ion from anode to intercalate into the cathode. 
     In the discharged state the battery includes a cathode characterized by the presence of A&#39; z  A y  MZ x  in which A&#39; is alkali metal more electronegative than A and z is a numerical value in the range 0&lt;z≦3.25.

This is a division, of application Ser. No. 943,107, filed Sept. 18,1978, now U.S. Pat. No. 4,206,276, granted June 3, 1980.

BACKGROUND OF THE INVENTION

This invention relates to the field of high energy batteries andparticularly batteries based on the intercalation compounds of layeredtransition metal dichalcogenides.

Batteries composed of intercalatable transition metal chalcogenides asthe cathode material are known in the art, as exemplified in U.S. Pat.No. 4,009,052 wherein are described alkali metal-based batteries usingintercalatable transition metal disulfides such as titanium disulfide ascathode. Such batteries are secondary batteries as distinguished fromprimary batteries in that they are capable of being charged anddischarged over many cycles. Specifically, these batteries arepredicated on the use of an intercalated layered dischalcogenide likeLi_(x) TiS₂ (in which x varies from 0 to 1) as the cathode-activematerial and lithium is intercalated or removed to and from the cathoderespectively during discharging and charging of the battery. Thelimiting discharge condition of the battery of the said patent isreached when x approaches 1 as a value.

Thus, there is a need to increase the capacity of the dischalcogenide tointercalate the alkali metal which is transported through the cell tothe cathode in order to realize higher capacity of the battery,primarily to obtain greater power from such batteries.

The present invention provides substantially higher capacity than thatrealized with the batteries of U.S. Pat. No. 4,009,052. For example, thecapacity of batteries in accordance with the present invention is up tothree times (per mole of the dichalcogenide) that of comparablebatteries prepared in accordance with the aforesaid U.S. Pat. No.4,009,052.

THE INVENTION

The present invention is based on the discovery that a partially alkalimetal intercalated dichalcogenide cathode-active material has a greatercapacity for anode alkali metal which is more electronegative and ofsmaller ionic size than the partially intercalated alkali metal, and theenhanced capacity of the said cathode-active material results in asubstantial increase in power of batteries incorporating same incomparision to the power of batteries produced in accordance with theprior art, particularly U.S. Pat. No. 4,009,052.

Specifically, it has now been discovered that the use of a partiallyalkali metal (e.g. sodium) intercalated dichalcogenide cathode-activematerial in conjunction with an anode of another alkali metal (e.g.lithium) which is more electronegative and smaller in ionic size thanthat partially intercalated in the cathode enhances the capacity of thecathode for the anode metal. Thus, where the partially intercalatedcathode material is of the formula A_(y) MZ_(x) in which A is an alkalimetal more electropositive than the anode metal, M is a transition metalof Group IV or V, Z is sulfur, tellurium or selenium, x is a numericalvalue of from about 1.8 to about 2.1 and y is a numerical value of fromabout 0.01 to about 1, the cathode-active material can be represented bythe formula A'_(z) A_(y) MZ_(x), wherein A' is a more electronegativeand smaller alkali metal than A, z is a numerical value in the range of0<z≦3.25 and A, Y, M, Z and x are as previously defined.

Thus the present invention provides for higher maximum intercalatedanode alkali metal than is possible utilizing the teachings of theaforementioned U.S. Pat. No. 4,009,052. In the latter, the maximumnumber of gram-atoms of intercalated anode metal is one, whereas in thepresent invention, the comparable number is about three, per mole ofdichalcogenide.

The invention will be described further in terms of the batterycomponents.

THE CATHODE

The cathode-active material of the battery consists of a partiallyintercalated transition metal dichalcogenide of the formula A_(y) MZ_(x)in which A is at least one alkali metal which is more electropositiveand larger in size that the anode-active metal, M is at least onetransition metal of Group IV or V, Z is at least one of sulfur,selenium, or tellurim, x has a numerical value of from about 1.8 toabout 2.1, and y has a numerical value of from about 0.01 to about 1,preferably from about 0.15 to about 0.2.

The cathode may consist entirely of the cathode-active material actingsimultaneously as the current collector as well. Alternatively, as inwell known in the art, the cathode may consist of the cathode activematerial supported on a suitable base structure such as stainless steel,nickel, carbon and similar support materials.

The anode-active alkali metals (A') include lithium, sodium, potassium,and rubidium with lithium being preferred. The partially intercalatedalkali metals (A) include solium, potassium, rubidium, and cesium, ormixtures thereof, with sodium being preferred with the preferred anode(lithium).

The transition metals include tantalum, titanium, zirconium, hafnium,niobium and vanadium, of these the preferred being titanium andtantalum. Mixtures of the transition metal chalcogenides are alsouseful, e.g. titanium-tantalum sulfide. In general, sulfides arepreferred of the chalcogenides particularly those wherein the value of xin the formula MZ_(x) is aoout 2.

A typical cathode composition is Na₀.15-0.2 TiS₂, e.g. Na₀.177 TiS₂.

THE ANODE

The anode is comprised of alkali metals which are more electronegativeand smaller in size than the partially intercalated alkali metal of thecathode-active material. Alloys of the alkali metals can be employed asanode, such as alloys with aluminum and silicon, as well as with otheralkali metals. The anode-active alkali metals include Li, Na, K and Rb,and preferably is lithium. Thus, the anode-active alkali metal can belithium when the partially intercalated alkali metal is sodium or aheavier alkali metal. Of course, the anode can be Na or K, if thepartially intercalated alkali metal is Rb or Cs.

As with the cathode, the anode may be comprised entirely of the alkalimetal, or it may be comprised of the alkali metal on a suitable supportlike nickel, aluminum, stainless steel or similar supports.

A typical anode is lithium when the cathode is an intercalate of sodium.

THE ELECTROLYTE

The electrolyte comprises a medium containing anode metal ions which isphysically and chemically compatible with both the anode and cathode ofthe battery. It may be solid or liquid offering rapid transport ofanode-active metal ions to and from the cathode, respectively, indischarging and charging cycles, i.e., during intercalation and removalof anode metal from the partially alkali metal-intercalated transitionmetal dichalcogenide.

Liquid electrolyte systems, which are usually preferred, are prepared bydissolving of the selected salt in suitable solvents. Typically, saltsinclude alkali metal (A') salts such as A'ClO₄, A'PF₆ and other similarsalts known to the art wherein A' is the anode-active alkali metal.Mixtures of electrolytic salts can be employed as is common. Thesolvents for such electrolytic systems include porpylene carbonate,tetrahydrofuran, dioxane, dioxolane, dimethoxyethane, ethylene carbonateand like solvents. The solvent may be employed alone or in mixture withother solvents, or inert diluent solvents. Both the electrolyte andsolvent therefor are well-known in this art.

A typical liquid electrolyte system can be prepared by dissolvinglithium perchlorate in propylene carbonate.

In commonly assigned copending application Ser. No. 891,807, filed Mar.30, 1978 now U.S. Pat. No. 4,132,837, there is described the improvementin the performance of organic aprotic-based electrolytes mentionedherein by incorporation of crown ethers (macroheterocyclic compounds)into the electrolytes. These additives can be employed in preparing thecell electrolyte for the present invention and the disclosure of thesaid copending application is incorporated herein by reference.

It is also within the scope of the invention to use electrolytes in thesolid, or fused state. For example, for the solid state, and "aluminacan be used whereas," for fused electrolyte, alkali metal halides can beemployed, e.g., Licl-Kcl mixtures.

The foregoing description, of course, refers to the anode, cathode, andelectrolyte in the charged state, i.e. on assembly as an electrochemicalcell, it will discharge.

The novel cathode of the present invention in the charged state ischaracterized by the presence of the partially alkali metal-intercalateddichalcogenide and can be used in any arrangement with a suitable anode,i.e. an anode of an alkali metal which is more electronegative andsmaller in size than the alkali metal of the cathode-active material.Thus the present high-energy density cathode can be used forelectrochemical cells in a variety of physical arrangements inconjunction with the suitable anode as long as provision is made fortransport of the anode metal ions to the cathode in discharge and fromthe cathode in charging. The resulting batteries show a high energyoutput compared to known batteries by virtue of the higher capacity ofthe cathode for the anode metal, as hereinbefore described.

It may be convenient to produce a cathode for a battery so that thebattery is in the discharged state in which case the anode is comprisedof the mixed alkali metal-intercalated dichalcogenide, A'_(z) A_(y)MZ_(x), wherein

A and A' are each alkali metal with A being more electropositive andlarger than A'

M=transition metal of Group IV or V

Z=S, Se or Te

x=numerical value from about 1.8 to 2.1

y=numerical value from about 0.01 to 1

z=numerical value in the range 0<z≦3.25.

While the concentration (z) of the second alkali metal (A') in the mixedalkali metal dichalcogenide can be any value greater than zero up toabout 3.25, i.e., the range 0<z≦3.25, normally, a concentration of atleast about 1×10⁻³ is preferred for most uses. On charging, thecathode-active material is generated by transfer of alkali metal A' fromcathode to anode, leaving the requisite A_(y) MZ_(x) as the residualmaterial of the cathode for discharge. The anode for the dischargedbattery can be simply a current-collecting structure which serves assupport for the anode alkali metal, as described hereinbefore. Thesupport, on charging of the battery, should be capable of receiving theanode alkali metal. Where the mixed alkali metal-intercalateddichalcogenide is fully intercalated, sufficient anode alkali metalwould be generated for the anode to make it unnecessary that the anodecontain additional anode alkali metal, although the anode can beprovided with such additional alkali metal, as desired.

The electrolyte and electrolyte system for the battery in the dischargedstate can be any of the electrolytes and electrolyte systems employed inthe battery in the charge state, as described hereinbefore.

For commercial production, in particular, it is convenient to assemblethe battery in the discharged state since such assembly obviates adifficulty inherent in the use of alkali metals as the anode materialdue to sensitivity to moisture and oxygen of the atmosphere (normallyrequiring inert, dry atmosphere for assembly).

The novel and useful mixed alkali metal-intercalated dichalcogenide isespecially valuable in permitting the assembly of the new high energydensity cells of the present invention. These new products arepreparable by various procedures, including electrolytic methods whereinA_(y) MZ_(x) is employed as cathode and a source of A' is present toresult in transfer of A' to the cathode active material. Additionally,the present new products can be prepared by reaction of A_(y) MZ_(x) insuitable solvents with alkali metal (A') derivatives such as butyllithium, sodium naphthalide, and similar such compounds. The mixturesare allowed to stand for extended periods, e.g., one to several days,after which the products are recovered from the solvent afterfiltration. Alternatively, these products can be prepared from thedichalcogenide MZ_(x) in a two-step reaction in the first step of whichthe alkali metal (A) is reacted with MZ_(x) in a reaction solvent, usingselected amounts of the A alkali metal compound, (e.g., Na naphthalide)to form the intercalated dichalcogenide, A_(y) MZ_(x), which is thenseparated in solvent from the reaction mixture and reacted with theaforesaid A' alkali metal derivative to form the products of theinvention.

The stoichiometry of the initial reaction to form A_(y) MZ_(x) must becontrolled to provide y atoms of A in the intermediate. The productA_(y) MZ_(x) is easily analyzed to determine the value of y usingart-recognized procedures, e.g., titration with standard acid.

The following examples further illustrate the invention:

EXAMPLE 1

Tantalum disulfide was annealed at 580° C. for 7 days in the presence ofsulfur vapor in a stream of argon. This procedure converted tantalumdisulfide close to the stoichiometric formula, TaS₂, having a2H-polytype structure.

About 11.2 g of this powder was intercalated with sodium by bringing itinto contact with sodium naphthalide in tetrahydrofuran. The resultingcomposition of the sodium intercalated product was Na₀.177 TaS₂. Theintercalation and all subsequent operations were carried out under anargon atmosphere.

Five cells were constructed using the powder product as cathode-activematerial. For this purpose, the powder was pressed in a steel die into0.25 inch diameter pellets weighing 116, 155, 80, 120 and 96 mg.,respectively.

In the construction of each cell, one of these pellets was used ascathode by placing the pellet between a porous nickel felt metal discand a stainless steel rod of 0.25 inch diameter, electrically connectedto the cell cathode terminal of the measuring instrument.

The cell anode was made up of freshly cut lithium metal pieces of about0.25 inch diameter in a stainless steel block.

ELECTROLYTE SYSTEM

Propylene carbonate was shaken with freshly cut lithium pieces of smallsize, (250 ml. of solvent, and 2-3 g lithium pieces (freshly cut) of 1-5mm² surface area per piece: shaking was for 3 weeks). Filtration of thefragmented lithium pieces gave oxygen and moisture-free propylenecarbonate.

Anhydrous lithium perchlorate (dried under vacuum at 140° C. for 7 days)was dissolved in the propylene carbonate at 0.7 mole/liter for the cellelectrolyte.

THE DISCHARGE CHARACTERISTICS OF THE CELL

All the 5 cells were discharged to different extents by passing acurrent 0.315 mA/cm² of cathode area through the cells over variousperiods of time, after which they were allowed to relax and the voltagesrecorded. The extent of discharge, which is indicated by the extent ofintercalation of lithium into the cathode is expressed by thecomposition parameter, z, in Li_(z) Na₀.177 TaS₂.

Table I shows the various voltage values corresponding to the extent ofdischarge expressed as numerical value of z. The same table shows thevoltage values of other cathodes like Li_(z) TaS₂ and Li_(z) TiS₂(described in U.S. Pat. No. 4,009,052)as a function of z.

The data for the present invention clearly establishes the superiorityof the Li_(z) Na₀.177 TaS₂ cathode over that of U.S. Pat. No. 4,009,052in intercalating more than 3 times lithium per mole of disulfide. Thecathode of the present invention improves the capacity of the lithiumbattery significantly over the Li/TiS₂ cathode of U.S. Pat. No.4,009,052.

After being discharged, the cathode of the cells were charged afterwardsup to z=0.2 in Li_(z) Na₀.177 TaS₂ and discharged again. Therechargibility of the cathode was excellent and same voltage values werereproduced corresponding to specific values of z.

Because of the relative differences in the extent of electronegativityof sodium ion and lithium ion, during charging of the cell, lithiumcomes out of the cathode and not sodium, up to the extent of chargerepresented by z=0. Even if the charging is continued beyond z=0, whichwould lead to the expulsion of sodium ions from the cathode, duringsubsequent discharging of the battery, all the sodium would beintercalated first into the cathode (because of its lowerelectronegativity than lithium) before any lithium is intercalated. Thatwould in effect regenerate the Na_(y) TaS_(z) cathode with which thebattery was set up initially.

                  TABLE I                                                         ______________________________________                                        Extent    Voltage                                                             of discharge                                                                            (±0.1)                                                           expressed This     Voltage      Voltage                                       as `z`    Invention                                                                              Li/Tis.sub.2 Li/Tas.sub.2                                  ______________________________________                                        0.2       2.48     2.68         2.62                                          0.6       2.18     2.37         2.04                                          1.0       2.06     2.05         1.46                                          1.4       2.04     Discharge    Discharge                                                        does not go  does not go                                                      beyond z = 1.0                                                                             beyond z = 0.1                                1.8       2.00     Discharge    Discharge                                                        does not go  does not go                                                      beyond z = 1.0                                                                             beyond z = 1.0                                2.2       1.96     Discharge    Discharge                                                        does not go  does not go                                                      beyond z = 1.0                                                                             beyond z = 1.0                                2.6       1.92     Discharge    Discharge                                                        does not go  does not go                                                      beyond z = 1.0                                                                             beyond z = 1.0                                3.0       1.80     Discharge    Discharge                                                        does not go  does not go                                                      beyond z = 1.0                                                                             beyond z = 1.0                                ______________________________________                                    

EXAMPLE II

Stoichiometric TiS₂ (Cerac/Pure Inc., Butler, Wis.) was intercalatedwith an aliquot of sodium naphthalide in tetrahydrofuran by allowing themixture to stand for 2 days at room temperature under dry, inertatmosphere to obtain Na₀.177 TiS₂. Titration with standard acid was usedto determine the sodium content.

Thereafter, the Na₀.177 TiS₂ was employed in a battery as described inExample I for the analogous tantalum compound with comparable results asshown in Table II.

                  TABLE II                                                        ______________________________________                                        Li.sub.z Na.sub.0.177 TiS.sub.2 System                                        Extent of Discharge                                                           expressed as `z`                                                                              Voltage (±0.1) (Volts)                                     ______________________________________                                        0.623           2.46                                                          1.023           2.26                                                          1.33            2.27                                                          1.726           1.98                                                          2.49            2.05                                                          2.773           1.85                                                          ______________________________________                                    

In the foregoing examples, the electrolytic solvent system employed isfirst equilibrated with the anode-active metal particularly lithiummetal, to remove moisture and oxygen. When solvents so-treated areemployed in an electrochemical cell, higher voltage is realizedapparently due to improved anode performance. In comparison, the use ofsuch solvents when treated by standard techniques such as with activatedalumina treatment, distillation under reduced pressure and the like isnot accompanied by the improved anode performance as evidenced by lowervoltage values. Accordingly, the treatment of solvent with anode metalprior to use in the electrochemical cells of this invention constitutesa particularly preferred embodiment.

In the foregoing disclosure, reference has been made to alkali metals asthe preferred anode-active materials and as the preferred partialintercalate. It is also intended that other electrochemically-activemetals can be employed in the present invention, with the proviso thatthey can perform as is required of alkali metals designated A and A'.Thus, combinations of such metals in lieu of the corresponding alkalimetals will of course result in suitable electrochemical cells inaccordance with the invention. Thus, combinations of metals from amongBe, Mg, Ba, Ca and Sr can be employed, with the more electropositive andlarger serving as the "A" component of the system and the lesselectropositive and smaller metal serving as the A' component.

Accordingly, a suitable system will comprise magnesium as the anode andtitanium disulfide intercalated with calcium.

In the preferred form of the invention, the cathode-active material isthe mixed alkali metal intercalated dichalcogenide, A'_(z) A_(y) MZ_(x).The amount of anode metal intercalated is thus given by the value of z.

"z" is a numerical value which is based on the remaining number ofavailable sites for A', the anode-active metal, in the VanderWaal's gapsof the dichalcogenide after intercalation of A. The number of such siteswill be predicated on the nature of the alkali metals A and A'. Forexample, up to 3.25 lithiums are accommodated in the dichalcogenideintercalated with sodium at a value of about 0.2. The maximum value of zwill of course vary with the value of y (i.e., the concentration of Aalkali metal) and the size of alkali metal A and can assume valuesgreater than 3.25.

What is claimed is:
 1. An electrochemical cell in the charge state, saidcell having a cathode-active material, an anode-active material and anelectrolyte, wherein the cathode-active material is an alkali metalcomprising a partially alkali metal intercalated dichalcogenide of theformula A_(y) MZ_(x) whereinA is an alkali metal more electropositivethan the anode active material; M is at least one transition metal ofGroup IV or V; Z is selenium, sulfur or tellurium; y is a numericalvalue from about 0.01 to about 1; x is a numerical value from about 1.8to about 2.1; and, the anode-active material is an alkali metal.
 2. Acell according to claim 1 wherein A is sodium, M is titanium, and Z issulfur.
 3. A cell according to claim 1 wherein A is sodium, M istantalium and Z is sulfur.
 4. A cell according to claim 2 in pelletform.
 5. A cell according to claim 2 wherein y is a numerical value fromabout 0.15 to about 0.2.
 6. A battery in the charged state, said batteryhaving a cathode-active material, an anode-active material and anelectrolyte, wherein the cathode-active material comprises a partiallyalkali metal intercalated dichalcogenide of the formula A_(y) MZ_(x)whereinA is an alkali metal more electropositive and larger than theanode active material; M is at least one transition metal of Group IV orV; Z is selenium, sulfur or tellurium; y is a numerical value from about0.01 to about 1; x is a numerical value from about 1.8 to about 2.1 andthe anode-active material comprises an alkali metal.
 7. Anelectrochemical cell in the charge state, said cell having acathode-active material, an anode-active material and an electrolyte,wherein the cathode-active material is a metal intercalateddichalcogenide of the formula A_(y) MZ_(x) whereinA is a metal selectedfrom the group consisting of Be, Mg, Ba, Ca and Sr and is moreelectropositive and larger than the anode-active material; M is at leastone transition metal of Group IV or V; z is selenium, sulfur ortellurium; y is a numerical value from about 0.01 to about 1; and x is anumerical value from about 1.8 to about 2.1.
 8. A cell according toclaim 7 wherein A is calcium.
 9. A cell according to claim 7 wherein Ais magnesium.
 10. A cell according to claim 7 wherein M is tantalum. 11.A cell according to claim 7 wherein M is titanium.